The present invention relates generally to apparatuses and methods for cleaning thin discs or substrates, such as semiconductor wafers, compact discs, glass substrates, flat panel displays and the like. More particularly, the invention relates to a megasonic tank for cleaning semiconductor wafers.
Conventional megasonic cleaning tanks employ a fluid filled tank having substrate supports therein and a source of megasonic energy, (e.g., a transducer) coupled to the fluid for directing sonic energy through the fluid to the surfaces of a substrate or wafer supported therein. During megasonic cleaning, the transducer oscillates between a positive and a negative position at a megasonic rate so as to generate positive and negative pressures within the fluid, and thereby couple megasonic energy to the fluid. As the energy imparted to the fluid oscillates between positive and negative pressure, cavitation bubbles form in the fluid during negative pressure and collapse or shrink during positive pressure. This bubble oscillation and collapse gently cleans the surface of the wafer.
Particles cleaned from the wafer are carried upward via a laminar flow of fluid and flushed into overflow weirs coupled to the top of the cleaning tank. Thus, a supply of clean fluid is continually introduced to the cleaning tank from the bottom of the sidewalls thereof. Cleaning fluid distribution nozzles are positioned along the bottom of the sidewalls to supply various cleaning fluids through the same nozzles or through dedicated sets of nozzles.
Most conventional cleaning tanks position one or more transducers along the bottom of the cleaning tank. Acoustic waves from these transducers reflect from the surface of the cleaning fluid back into transducers, and interference results in reduced power density in the tank and reduced cleaning efficiency. Due to the limited area of the tank""s bottom, the number, size, placement and shape of the transducers, fluid inlets, etc., often can not be freely selected for optimal performance.
In practice, megasonic cleaners experience a number of other limitations as well. For instance, transducers with higher power density assure better cleaning efficiency, but generate considerable heat during operation. Accordingly, cooling systems are often used to prevent degradation of adhesive material that attaches a transducer to materials that couple the transducer""s acoustic power to the cleaning fluid and to prevent overheating of the power supply that could reduce the life of its electrical components. Such transducer cooling systems, however, undesirably increase the cost and complexity of a megasonic cleaning system.
An alternative approach has been to employ a cycled array of multiplexed transducers in which each transducer is on only 1/Nth of the cycle time, where N is the number of transducers per cleaning vessel. The reduction of duty cycle by a factor of N, which is usually 8 for 8 inch wafer batch processing vessels, reduces transducer temperatures and in some cases eliminates the need for transducer cooling systems. A major problem of this approach is the often unacceptable increase in processing time by a factor of N. The increase in processing time is particularly problematic for single wafer processing, where short processing time is an important requirement.
Another problem experienced by megasonic cleaners is the shadowing of the transducer""s acoustic field by the wafer supports on which a wafer is positioned. Because a support must be positioned below the wafer to stabilize the wafer, the supports directly block energy transmitted from transducers positioned below the wafer.
Accordingly, a need exists for an improved sonic cleaning tank that provides high laminar fluid flow yet avoids the interference of incident and reflected waves, maintains short processing times without requiring transducer cooling arrangements, and minimizes shadowing by substrate supports.
The present invention overcomes the shortcomings of the prior art by providing a sonic cleaning tank in which transducer arrays face the substrate from a plurality of angles, and are switched on and off alternately to provide cleaning energy across the entire surface of the substrate at any given time, yet enabling each transducer to experience a reduced duty cycle. The tank walls to which the transducers attach are angled upward and the tank is designed such that walls (if any) are positioned opposite a transducer so that energy reflected therefrom does not cross the surface of the substrate being cleaned (i.e., the walls are positioned for non-interfering reflection). Preferably, the lower side walls or the bottom wall of the cleaning tank are angled upward forming two or more upwardly angled walls. Arrays of one or more transducers are positioned along at least two of the two or more upwardly angled walls. Each transducer array is equal to or greater in length than the diameter of the substrate being cleaned so that alternately energizing the two or more transducer arrays provides nearly 100% cleaning across the substrate""s surface, yet requires only a 50% (or less depending on the number of transducer arrays) duty cycle for each transducer. Thus, the inventive arrangement is far superior to any known sonic tank-type cleaner.
Not only does the invention provide fast processing times (due to nearly 100% surface cleaning at any given time) and enable transducers and power supplies to last longer (due to reduced duty cycles) without the aid of complex cooling systems, the invention reduces or eliminates both substrate shadowing and interference from energy waves reflected back toward the transducer. Because each transducer array faces the substrate from a unique angle, substrate support shadowing can be eliminated with appropriate substrate support placement. For instance, by employing two side substrate supports and one bottom substrate support, and by positioning the bottom substrate support along the baricenter of the substrate, the bottom support will not shadow the same region of the substrate from the energy of both transducer arrays.
Similarly, because the energy from the two transducer arrays approaches the substrate from opposing directions, the two side supports can be positioned so as not to cause shadowing by blocking energy from the transducer array opposite thereto, e.g., by positioning the side supports along opposite sides of the substrate""s horizontal diameter. It will be understood that the portion of the substrate which is contacted by the substrate support will be blocked continuously from transducer energy. Thus, as used herein, shadowing refers to the blocking of transducer energy from those portions of the substrate which are not in contact with a substrate support.
Further, the inventive cleaning tank is advantageously configured such that any transducer energy which travels across the substrate, impacts an opposing sidewall, and reflects therefrom will travel upwardly to the air/liquid interface without crossing the substrate, and without interfering with the cleaning of the substrate""s surface (i.e., in a non-interfering manner). This is achieved by employing transducer arrays positioned on the upwardly angled walls, as will be understood with reference to the drawings provided herein. In a further embodiment, rotating the substrate causes each portion of the substrate""s beveled edge to pass through the regions of highest intensity cleaning, i.e., the regions closest to a transducer array, for example closest to the first transducer array, and closest to the second transducer array. Because a single rotation causes the substrate to pass through two or more high intensity cleaning regions, superior edge cleaning is achieved.
Accordingly the invention eliminates many of the parts required by conventional sonic cleaning tanks, prevents substrate shadowing, prevents interfering reflection and lengthens transducer life by reducing duty cycle, but does so without increasing processing time. other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.